Facilities and/or operations involving high volume air flows, for example wind tunnels, gas turbine engine test facilities, power generation facilities, industrial or manufacturing facilities, e.g., vehicle manufacturing and testing facilities, or any other facility that houses or uses a prime mover, typically move or flow large or massive amounts of air when in operation. Due to the air flow(s) and/or other processes, they may generate very high acoustic levels inside and outside the facility. Noise created by air flow is, among other mechanisms or causes, the result of shearing within the flow due to high velocity gradient in adjacent flow paths. Typically, air flowing both in and out of these facilities must be treated acoustically to maintain acceptable sound and/or noise levels, e.g. in the surrounding community, and the noise must be mitigated without excessive resistance and while maintaining uniform flow. Typically, sound absorbing or insulating structure(s) are used to absorb acoustical energy from the air flow. Such structures are generally required on both the inlet and exhaust side, and may be referred to and/or known as acoustical baffles. Additionally, facilities such as those mentioned above and/or others also typically require a well behaved interior air flow to maintain stable processes. Acoustical baffles may serve a dual purpose as they reduce noise and assist in maintaining conditioned interior air flow.
The general approach to the noise problem in facilities or situations such as those mentioned above by way of example may be to integrate a large array of absorptive baffles in inlet and/or exhaust segments. The shape, spacing, and effective length of these baffles are dictated by the specific frequency distribution and amplitude of the source noise as compared to the desired values outside the facility. To mitigate the higher acoustic energy levels long or thick baffles are generally required.
Historical implementations of acoustical baffles in gas turbine engine test facilities, for example, include installation of many large “slab-type” acoustical baffles. These baffles are in the shape of a rectangular prism and generally have aerodynamic features, such as triangular or hemispherical caps on the leading and trailing edges. The baffles typically have an internal skeletal structure forming partitions for absorptive acoustical material. The sides of the structures are clad with perforated steel material. The baffles are typically suspended vertically in inlet and exhaust flow streams in an orientation with the “slabs” or baffles aligned vertically with the direction of flow. Spacing between baffles and installed lengths are determined by the required aerodynamic and acoustical requirements of the facility, e.g. the test cell. A common problem with this type of baffle is their massive size which makes them very expensive to manufacture and difficult to install. Further, the spacing between baffles forms large segregated channels that partitions the air flow. This partitioning does not provide good mixing within the air flow or the potential for correction and/or adjustment of airflow distribution, if necessary, to produce a final total flow stream with a well behaved and uniform velocity distribution. Implementation of this type of baffle has resulted in, in addition to other undesirable phenomena: noise induced by the baffles themselves, ineffective noise reduction, and re-entrainment of exhaust air due to the significant differences in velocity in adjacent partitions.
Another difficulty with such known baffles is welding may be commonly employed to attach relatively thin perforated skins to the structures. With high vibration levels, these welds can be sources of failure due to local hardening adjacent to the weld and thermal stresses.
The use of “square bar silencers” to replace the slab-type baffles is known. Instead of installing rows of a few large slab-type baffles, a matrix or grid of smaller baffles in the shape of a square prism is installed in the air flow. The bars are suspended with the long direction in the direction of air flow. The dimensions of the square section, the length of the bar, and the spacing of bars are dependent on the noise attenuation and aerodynamic requirements of the test cell. The primary benefits of this type of configuration are lower cost of manufacture, installation and servicing, and more ease in “tuning” the performance of the baffle system by modifying the grid for optimum acoustical attenuation. Unlike slab-type baffles, this type of baffle does not partition the air flow so the air can “fill” the volume and normalize to a final flow with a small, uniform velocity distribution. The disadvantage of these baffles is still a high cost to manufacture. The four sided square also increases the surface area that can be installed in a given length of baffles. By reducing the installed length, building geometries are reduced and acoustic outer packaging is reduced. To be effective, the overall surface area of the absorptive baffles must be significant. This requires a large material content that also drives cost. While an improvement over slab-type baffles, they are still typically expensive to manufacture, install and maintain.